Wash. Admin. Code § 308-91-055
An owner must preserve the records on which the owner's application for apportioned registration is based for a period of three years following the close of the registration year. These records shall be complete and shall include, but not be limited to, the following: Copies of proportional registration applications and supplements for all jurisdictions in which the fleet is prorated; proof of proportional or full registration with other jurisdictions; vehicle license or trip permits; temporary authorization permits; documents establishing the latest purchase year and cost of each fleet vehicle in ready-for-the-road condition; weight certificates indicating the unladen, ready-for-the-road, weight of each vehicle in the fleet; periodic summaries of mileage by fleet and by individual vehicles; individual trip reports, driver's daily logs, or other source documents maintained for each individual trip that provide trip dates, points of origin and destinations, total miles traveled, miles traveled in each jurisdiction, routes traveled, vehicle equipment number, driver's full name, and all other information pertinent to each trip.
Wash. Admin. Code § 308-91-055